11.6 Oracle single instance vs. clustered configuration

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Every Oracle configuration starts with a single instance, in the sense that even in the clustered Oracle configuration like RAC, it starts with a single instance. From the basic level of database creation, database management, database performance tuning, etc., all operations start with a basic single instance configuration and move to a clustered configuration. It is very important in every aspect of database administration and maintenance that each instance is considered as an individual unit before considering it as a combined cluster.

Stand-alone, or single instance, configuration in an enterprise system does not provide all the functionalities discussed in Chapter 2, such as availability and scalability. One way of providing for availability is by using some of the high-availability options accessible from Oracle, for example, the ODG (previously called Hot Standby) where the data from the primary database is copied to the remote database. This is done by copying the redo logs when the redo log switch occurs. The difficulty with such a configuration is that there could be loss of data when the node that contains the primary database fails and the last set of redo logs are not copied over to the destination database. This creates an inconsistent environment.

Another high-availability option would be to use the OAR feature. This option is very similar to the ODG option; however, instead of copying the redo logs from the primary instance to the secondary, or target replicated environment, data could be transferred more frequently like a record, or a group of records. This feature, when compared to the ODG option, provides a much closer level of data consistency. This is due to the fact that in the case of failure of the node that contains the primary database, only the last few rows, or sets, of data are not transferred.

From a disaster recovery or reporting solution, the ODG and OAR feature are high-availability options. Where data consistency is not an immediate concern, such as in the case of disasters due to an ''act of God,'' where the primary database is not available, a remote database created by either of these options could help provide a backup opportunity to the enterprise system.

Oracle's clustered, or multi-instance configurations comprise multiple nodes working as a cohesive unit with each node in the cluster consisting of two or more instances talking to a common shared database. As has been discussed, this feature is the RAC configuration.

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